Sandia LabNews

Geothermal drilling gets boost with polyurethane grout

New hope for the geothermal energy industry comes in form of polyurethane grout

Members of Sandia’s Geothermal Research Department have added a new approach to aid in the often-difficult task of coaxing usable energy from the earth’s crust with a drill bit. Their secret: polyurethane grout. Specially formulated grouts — dense foam-like materials — can now be used to seal voids in fractured rock formations, allowing geothermal drilling to go forward.

A demonstration last year proved the concept can work. Now testing is proceeding to improve the materials and techniques for delivering them, says A. J. "Chip" Mansure (6211).

In the past, rock fractures and voids — termed "lost circulation zones" by drillers — have drained drilling mud away from wells, damaged drill bits, pinched off drilling pipe in the hole, and resulted in collapses and abandonment. These problems mean big bucks — industry-wide as much as 20 percent of geothermal drilling costs.

"Imagine drilling into a wall at home with a long, thin bit and a hand-crank drill," says Chip, who leads Sandia’s effort for developing the grout plugs. "If the bit chatters or bends or wobbles or sticks, it creates problems. In the geothermal industry, these problems translate to costs."

Beyond chewing away rocks with a drill bit, successful drilling must bring the rock fragments out of the well bore. To do this, "mud" solution is pumped down the inside of the drill pipe and out through the bit. The mud carries rock chips back up through the well bore to clear the hole. But in a fractured zone, the mud may leak away without bringing the rock chips up, Chip explains.

Heat and pressure — givens for geothermal areas — add to the problems of sealing these zones with most materials. And "cross flows," a condition where fluids enter a well bore from one fracture and leave through another, make it difficult to keep sealants in place long enough to harden.

Zones of lost circulation also make it difficult to cement casing in place in the well. Unlike oil and gas wells, where casing can be "tacked" to the rock formation at key points, the rigors of steam and heat in a geothermal well require that the bore be fully cemented.

Switching gears, changing plans

With colleague Jimmy Westmoreland, Chip had been working on a full-scale experiment to test polyurethane materials as a potential seal, when a phone call from Nevada changed their plans. "We knew that polyurethane had been tried years ago and failed," says Chip. "But we also knew that more recent work on dams and other civil engineering applications with different chemistry had been successful."

Instead of a full-scale test in Tech Area 3, Chip and his colleagues found themselves headed for Rye Patch, Nev. A geothermal well there had been designated by DOE for special funding to resolve a cross-flow problem that had resulted in abandonment of the well. "We took a breath, switched gears, and put a total project together," Chip says.

Past experience and laboratory testing had shown Chip that successful seals would result only if the loss zone was carefully isolated, or "packed off" in the well bore and then the polyurethane squeezed in to sweep out other debris and fill the void. This called for careful coordination in drilling out a temporary plug in the existing well and testing the new sealant. The Rye Patch project would need to involve the operators of the site, Mount Wheeler Power, the drilling contractor, drilling engineer, project managers, the chemical manufacturers and distributors, and other consultants and helpers. "This was technology transfer right from the beginning," says Chip.

Chip acted as project manager and involved George Staller (now retired) in the design of the project. Ron Jacobson was instrumental in fielding the equipment, and Jim Grossman (all 6211) provided software and programming support.

Brave new world of geothermal

The project, conducted in spring of 2001, succeeded in sealing the high cross-flow zone. Previously more than 20 attempts with other types of plugs had failed. In addition to the polyurethane grout, the effort used a special drilling rig, which removed rock cuttings with a stream of air, and a nitrogen foam cement to bond the casing to the rock.

"The Rye Patch well demonstrated the suitability of the polyurethane materials," says Chip. But much work remains to be done.

Changes in the geothermal industry as it matures are affecting technology development, Chip says. Past domination of geothermal production by large multi-resource energy companies has given way to smaller firms with a primary focus in geothermal. At the same time drilling is beginning to require deeper holes, because the known, easily recoverable resources have been exploited. "Drilling costs go up more or less exponentially as you drill deeper," Chip says.

Chip and his colleagues have developed a series of "best practices" as a template to help industry drillers and service companies make best use of the new plugging materials.

Targeting deeper wells

Up to 90 percent of lost-circulation problems are encountered in the early (shallow) going of geothermal wells — at depths of a few hundred feet and temperatures below the boiling point of water. But deeper targets may mean formulations of foam that can be injected at a few thousands of feet and temperatures nearly in the 400 degree F range. "We are now studying what techniques and practices we can use on these deeper targets," Chip explains.

Another issue is working within the economic constraints associated with "real world" drilling projects. "A high-cost chemical in an industrial setting like this isn’t going to be feasible," Chip says. "We are working with the manufacturers on cost-effective formulations and we will be testing these." It appears polyurethane plugs can be provided at about 30 percent of the cost of traditional lost circulation sealing methods.

While the Rye Patch project required many people on site as well as a great deal of pre-planning, a more typical application will have to be accomplished by a much smaller crew and on shorter notice. "We are working to design an approach to bring this process more in line with industry practices," says Chip.